The concept of using white liquor instead of pure sodium hydroxide in whole or in part in an alkali extraction step in cellulose bleaching is well known. In order to eliminate the risk of hydrogen sulfide formation when using white liquor in a bleaching step at a pH below 10 and to permit temperature control in the bleaching step, it has been suggested that the white liquor should be oxidized in equipment similar to a black liquor oxidation plant, that is to provide a long contact time between gas and liquid. However, and so far as is known, white liquor oxidation processes are not similar to the black liquor for a number of reasons.
The concept of white liquor oxidation is a natural development following the commercialization of oxygen delignification. The use of white liquor as an alkali source in oxygen bleaching requires that the sodium sulfide be oxidized prior to the delignification reactor. This is necessary since the oxygen atmosphere in the reactor causes the sodium sulfide to oxidize.
As those skilled in the art are aware, the oxidation of sodium sulfide is slow and requires a catalyst. In black liquor oxidation the reaction is catalyzed by the organics normally found in the liquor. Since these organics are not present in white liquor a small amount of black liquor is normally added to the incoming white liquor. Furthermore, the reaction rate of white liquor is much more dependent on the sulfide concentration, the oxygen concentration and the temperature.
Since pulp mills are shifting to oxygen bleaching to minimize dioxin content in the product and mill effluent, it is necessary to have a caustic source. But inasmuch as caustic soda is expensive it becomes feasible to reclaim the NaOH values from white liquor. However, white liquor contains sodium sulfide which reacts exothermically with the oxygen to initiate a reaction that is difficult to control.
Among the known prior art is a method of white liquor oxidation shown in U.S. Pat. No. 4,053,352 to Hultman. This patent discloses a system in which white liquor is oxidized with air at an elevated temperature to convert all sulfides to thiosulfates, thereby enabling the white liquor to be used in a number of steps including oxygen bleaching. No reactor of specific design is shown and in any event does not disclose a method such as is herein described and claimed.
In U.S. Pat. Nos. 3,928,351 and 3,997,300 to Boatwright, a two stage black liquor oxidation process is described in which air is introduced to inner and outer section of a reaction vessel through which black liquor is circulated. The reaction vessel is significantly different in structure from that of the instant invention.
U.S. Pat. No. 3,655,343 to Galeano shows a dual stage oxidation apparatus for black liquor having first and second oxidation chambers. The reaction chambers of this apparatus are connected to one another in series in which the spent liquor is first atomized and mixed with oxygen. The first oxidized liquor is then reoxidized and again mixed with molecular oxygen to complete the oxidation process. Again, this system is significantly different and does not pertain to white liquor. U.S. Pat. Nos. 4,239,589 to Elton; 4,255,848 to Sato; 3,654,070 to Pradt; and 3,362,868 to Backlund also teach systems and apparatus to oxidizing spent digestion liquors. However, none of these patents shows a method or reaction vessel structure which remotely similar to the system of this invention.